In order to exploit the elevated methionine dependence of tumors for therapy, recombinant methionine alpha, gamma-lyase (rMETase) has been cloned, overexpressed and purified by our laboratory. The rMETase IC50 for a wide range of tumor cell lines of all major cancer types is significantly lower than that of normal cells tested. rMETase is active as a single agent against human tumors growing in nude mice, and is synergistic with 5-FU and CDDP. Initial Phase I clinical studies have demonstrated that a single administration of rMETase depleted serum methionine levels down to 0.1% (0.1 muM) of baseline. In order to prevent immunological reactions which might be produced by multiple dosing of rMETase, a bacterially-derived protein, and to prolong the half-life of rMETase, rMETase has been initially coupled to polyethylene glycol (PEG). Preliminary results have demonstrated that PEG- rMETase can significantly reduce antigenicity and increase serum half-life, while maintaining potency. In the present application, optimization of PEGylation will be carried out by varying the ratio of PEG linkers and rMETase as well as the length of the reaction. Five different candidate PEG linkers will be evaluated. The PEG-rMETases with the longest serum half-life and methionine depletion period will be chosen for evaluation in active systemic anaphylaxis (ASA) studies in guinea pigs. The MTD and efficacy of PEG-rMETase and the combinations of PEG-rMETase and 5-FU or CDDP will be determined with in metastatic human colon and lung cancer nude mouse models. Mouse survival time will be evaluated as the main endpoint. Pre-clinical studies including acute and sub-acute toxicity studies of PEG- rMETase and further efficacy studies on various human tumor types will be completed in Phase II. PROPOSED COMMERCIAL APPLICATIONS: PEG-rMETase should be useful alone or in combination with other types of widely-used anti-neoplastic agents, providing a significant commercial potential.